4. Construction of drainage structures

4.1 Shape of structures

Small-diameter drainage pipes are generally circular. The ovoid shape, which allows higher velocity for the same gradient and flow rate, is only used for larger structures.

In network headers, the diameter cannot be adapted to the flow rate (the slope being more or less fixed by the topography). Minimum diameters are determined to limit the risk of obstruction:

• 0.30 m for storm runoff ;

• 0.20 m for wastewater.

The 0.15 m diameter often used has been abandoned, except for private connections.

The upper diameter limit is high (theoretically 0.60 m), as the circular shape is well suited to factory prefabrication,...